github.com/thanos-io/thanos@v0.32.5/docs/proposals-rejected/201807-store-instance-high-availability.md (about)

     1  ---
     2  type: proposal
     3  title: High-availability for store instances
     4  status: rejected
     5  owner: mattbostock
     6  menu: proposals-rejected
     7  ---
     8  
     9  ## Summary
    10  
    11  This proposal makes total sense and solves our goals when using gossip. However there exists a very easy solution to this problem in form of using just static entry with any loadbalancer like Kubernetes Service to load balance through different Store Gateways. Those are technically stateless, so request can fetch the data independently.
    12  
    13  ## Motivation
    14  
    15  Thanos store instances currently have no explicit support for high-availability; query instances treat all store instances equally. If multiple store instances are used as gateways to a single bucket in an object store, Thanos query instances will wait for all instances to respond (subject to timeouts) before returning a response.
    16  
    17  ## Goals
    18  
    19  - Explicitly support and document high availability for store instances.
    20  
    21  - Reduce the query latency incurred by failing store instances when other store instances could return the same response faster.
    22  
    23  ## Proposal
    24  
    25  Thanos supports deduplication of metrics retrieved from multiple Prometheus servers to avoid gaps in query responses where a single Prometheus server failed but similar data was recorded by another Prometheus server in the same failure domain. To support deduplication, Thanos must wait for all Thanos sidecar servers to return their data (subject to timeouts) before returning a response to a client.
    26  
    27  When retrieving data from Thanos bucket store instances, however, the desired behaviour is different; we want Thanos use the first successful response it receives, on the assumption that all bucket store instances that communicate with the same bucket have access to the same data.
    28  
    29  To support the desired behaviour for bucket store instances while still allowing for deduplication, we propose to expand the [InfoResponse Protobuf](https://github.com/thanos-io/thanos/blob/b67aa3a709062be97215045f7488df67a9af2c66/pkg/store/storepb/rpc.proto#L28-L32) used by the Store API by adding two fields:
    30  
    31  - a string identifier that can be used to group store instances
    32  
    33  - an enum representing the [peer type as defined in the cluster package](https://github.com/thanos-io/thanos/blob/673614d9310f3f90fdb4585ca6201496ff92c697/pkg/cluster/cluster.go#L51-L64)
    34  
    35  For example;
    36  
    37  ```diff
    38  --- before	2018-07-02 15:49:09.000000000 +0100
    39  +++ after	2018-07-02 15:49:13.000000000 +0100
    40  @@ -1,5 +1,6 @@
    41   message InfoResponse {
    42     repeated Label labels = 1 [(gogoproto.nullable) = false];
    43     int64 min_time        = 2;
    44     int64 max_time        = 3;
    45  +  string store_group_id = 4;
    46  +  enum PeerType {
    47  +    STORE  = 0;
    48  +    SOURCE = 1;
    49  +    QUERY  = 2;
    50  +  }
    51  +  PeerType peer_type    = 5;
    52   }
    53  ```
    54  
    55  For the purpose of querying data from store instances, stores instance will be grouped by:
    56  
    57  - labels, as returned as part of `InfoResponse`
    58  - the new `store_group_id` string identifier
    59  
    60  Therefore, stores having identical sets of labels and identical values for `store_group_id` will belong in the same group for the purpose of querying data. Stores having an empty `store_group_id` field and matching labels will be considered to be part of the same group. Stores having an empty `store_group_id` field and empty label sets will also be considered part of the same group.
    61  
    62  If a service implementing the store API (a 'store instance') has a `STORE` or `QUERY` peer type, query instances will treat each store instance in the same group as having access to the same data. Query instances will randomly pick any two store instances[1](https://www.eecs.harvard.edu/~michaelm/postscripts/mythesis.pdf) from the same group and use the first response returned.
    63  
    64  Otherwise, for the `SOURCE` peer type, query instances will wait for all instances within the same group to respond (subject to existing timeouts) before returning a response, consistent with the current behaviour. This is necessary to collect all data available for the purposes of deduplication and to fill gaps in data where an individual Prometheus server failed to ingest data for a period of time.
    65  
    66  Each service implementing the store API must determine what value the `store_group_id` should return. For bucket stores, `store_group_id` should contain the concatenation of the object store URL and bucket name. For all other existing services implementing the store API, we will use an empty string for `store_group_id` until a reason exists to use it.
    67  
    68  Multiple buckets or object stores will be supported by setting the `store_group_id`.
    69  
    70  Existing instances running older versions of Thanos will be assumed to have an empty string for `store_group_id` and a `SOURCE` peer type, which will retain existing behaviour when awaiting responses.
    71  
    72  ### Scope
    73  
    74  Horizontal scaling should be handled separately and is out of scope for this proposal.
    75  
    76  ## User experience
    77  
    78  From a user's point of view, query responses should be faster and more reliable:
    79  
    80  - Running multiple bucket store instances will allow the query to be served even if a single store instance fails.
    81  
    82  - Query latency should be lower since the response will be served from the first bucket store instance to reply.
    83  
    84  The user experience for query responses involving only Thanos sidecars will be unaffected.
    85  
    86  ## Alternatives considered
    87  
    88  ### Implicitly relying on store labels
    89  
    90  Rather than expanding the `InfoResponse` Protobuf, we had originally considered relying on an empty set of store labels to determine that a store instance was acting as a gateway.
    91  
    92  We decided against this approach as it would make debugging harder due to its implicit nature, and is likely to cause bugs in future.
    93  
    94  ### Using boolean fields to determine query behaviour
    95  
    96  We rejected the idea of adding a `gateway` or `deduplicated` boolean field to `InfoResponse` in the store RPC API. The value of these fields would have had the same effect on query behaviour as returning the peer type field as proposed above and would be more explicit, but were specific to this use case.
    97  
    98  The peer type field in `InfoResponse` proposed above could be used for other use cases aside from determining query behaviour.
    99  
   100  ## Related future work
   101  
   102  ### Sharing data between store instances
   103  
   104  Thanos bucket stores download index and metadata from the object store on start-up. If multiple instances of a bucket store are used to provide high availability, each instance will download the same files for its own use. These file sizes can be in the order of gigabytes.
   105  
   106  Ideally, the overhead of each store instance downloading its own data would be avoided. We decided that it would be more appropriate to tackle sharing data as part of future work to support the horizontal scaling of store instances.